The present invention is in the field of battery technology and, more particularly, in the area of solid polymeric materials and composites for use in electrodes and electrolytes in electrochemical cells.
Conventional lithium ion batteries include a positive electrode (or cathode as used herein), a negative electrode (or anode as used herein), an electrolyte, and, frequently, a separator. The electrolyte typically includes a liquid component that facilitates lithium ion transport and, in particular, enables ion penetration into the electrode materials.
In contrast, so-called solid-state lithium ion batteries do not include liquid in their principal battery components. Solid-state batteries can have certain advantages over liquid electrolyte batteries, such as improvements in safety because liquid electrolytes are often volatile organic solvents. Solid-state batteries offer a wider range of packaging configurations because a liquid-tight seal is not necessary as it is with liquid electrolytes.
Generally, batteries having a solid-state electrolyte can have various advantages over batteries that contain liquid electrolyte. For small cells, such as those used in medical devices, the primary advantage is overall volumetric energy density. For example, small electrochemical cells often use specific packaging to contain the liquid electrolyte. For a typical packaging thickness of 0.5 mm, only about 60% of the volume can be used for the battery with the remainder being the volume of the packaging. As the cell dimensions get smaller, the problem becomes worse.
Elimination of the liquid electrolyte facilitates alternative, smaller packaging solutions for the battery. Thus, a substantial increase in the interior/exterior volume can be achieved, resulting in a larger total amount of stored energy in the same amount of space. Therefore, an all solid-state battery is desirable for medical applications requiring small batteries. The value is even greater for implantable, primary battery applications as the total energy stored often defines the device lifetime in the body.
Further, soft-solid state batteries can use lithium metal as the anode, thereby dramatically increasing the energy density of the battery as compared to the carbon-based anodes typically used in liquid electrolyte lithium ion batteries. With repeated cycling, lithium metal can form dendrites, which can penetrate a conventional porous separator and result in electrical shorting and runaway thermal reactions. This risk is mitigated through the use of a solid nonporous electrolyte.
The electrolyte material in a soft-solid-state lithium ion battery can be a polymer. Suitable polymers have the ability to conduct lithium ions. The solid electrolyte is typically formulated by adding a lithium ion salt to the polymer in advance of building the battery, which is a formulation process similar to liquid electrolytes.
However, solid-state batteries have not achieved widespread adoption because of practical limitations. For example, while polymeric solid-state electrolyte materials like poly(ethylene oxide) (“PEO”) are capable of conducting lithium ions, their ionic conductivities are inadequate for practical power performance. Successful solid-state batteries require thin film structures, which reduce energy density, and thus have limited utility.
Certain embodiments of the invention disclosed herein provide novel formulations for solid-state electrode films.